1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) panel, and more particularly, to a liquid crystal display panel implementing a bistable liquid crystal and a method of fabricating the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for improving a response time as well as reducing power consumption in the LCD panel.
2. Discussion of the Related Art
Recently, a liquid crystal display (LCD) panel with light, thin, and low power consumption characteristics is used in office automation equipment and video units. Such LCD's typically utilize an optical anisotropy of a liquid crystal (LC). Thin and long molecules of the LC cause an orientation alignment of the molecules. Therefore, applying electric fields to the molecules controls an alignment direction of the LC. When the alignment direction of the molecules in the LC is properly adjusted, the LC is aligned so that light is refracted along the alignment direction of the LC molecules. AS a result, image data is diplayed on the panel.
Currently, an active matrix (AM) LCD having a plurality of thin film transistors and pixel electrodes therein in the shape of an array matrix draws great attention. This is due to its high resolution and superiority in displaying moving pictures. Driving methods for such LCD's typically include a twisted nematic (TN) mode and a super twisted nematic (STN) mode.
A conventional liquid crystal display (LCD) device includes upper and lower substrates that are attached with each other, and an interposed liquid crystal, such as nematic, smectic, and cholesteric liquid crystals. A liquid crystal display device utilizes an electro-optic effect of the liquid crystal. A display panel consists of a plurality of liquid crystal cells. On the exterior surfaces of the upper and lower substrates, polarizers or retardation films are selectively attached.
FIG. 1 is a cross-sectional view illustrating a conventional LCD panel 20. As shown in FIG. 1, the LCD panel has lower and upper substrates 2 and 4, and an interposed liquid crystal layer 10. The lower substrate 2 includes a substrate 1, a TFT “S” as a switching element to change an orientation of liquid crystal molecules, and a pixel electrode 14 where a voltage is applied to the liquid crystal layer 10 in accordance with signals from the TFT “S”. The upper substrate 4 has a color filter 8 for implementing color. A common electrode 12 on is further formed the color filter B. The common electrode 12 serves as an electrode for applying a voltage to the liquid crystal layer 10. The pixel electrode 14 is arranged over a pixel portion “P”, i.e., a display area. Further, to prevent leakage of the liquid crystal layer 10 between the substrates 2 and 4, the substrates 2 and 4 are sealed by a sealant 6. The nematic, smectic, and cholesteric liquid crystals are most widely used in the above-mentioned LCD panel.
FIGS. 2A to 2C illustrate various types of liquid crystal molecules in the liquid crystal layer. More specifically, FIG. 2A is a schematic view of the nematic liquid crystal having each rod-like molecule that fluctuates quite rapidly. The molecules of the nematic liquid crystal have a definite orientation order expressed by a unit vector “{hacek over (n)}” called a director. The smectic liquid crystal is shown in FIG. 2B. The molecules of the smectic liquid crystal have a layered structure in which the molecular orientation is perpendicular or nearly perpendicular to the layer. As shown in FIG. 2C, in the cholesteric liquid crystal, the director {hacek over (n)} changes its orientation gradually along a helical axis. The helical axis coincides with the optical axis in the cholesteric liquid crystal. Among the three different types of liquid crystals, the nematic liquid crystal is most widely used in liquid crystal display devices because it has the strongest dispersion characteristic.
In general, liquid crystals for liquid crystal display panel must have the following characteristics. First of all, a the liquid crystals should maintain its phase in the range of low to high temperatures, so that it is operable in the wide temperature range. Also, liquid crystals should be chemically and optically stable for a long period of time. They should have a low viscosity and a fast response time. In addition, they must have highly ordered molecular alignments, thereby providing a high contrast ratio. Further, the liquid crystals should have a large dielectric anisotropy and a low operating voltage.
An electro-optic effect enables electrical modulation of light by changing an alignment of the liquid crystal molecules through the applied electric fields. The electro-optic effect of the liquid crystals is caused by the electrical modulation of light when an optical characteristic of the liquid crystals changes. The electrical modulation occurs when an alignment change of the liquid crystal molecules by applying electric fields to the liquid crystal.
Among the various types of nematic liquid crystals, a twisted nematic (TN) liquid crystal and a super twisted nematic (STN) liquid crystal are mostly used. For the TN LCD panel, a nematic liquid crystal is interposed between lower and upper transparent electrodes (shown as the common electrode 12 and the pixel electrode 14 in FIG. 1). Those electrodes are respectively treated to induce a certain molecular arrangement, so that a gradual rotation of the molecules occurs between the lower transparent electrode and the upper transparent electrode until a twist angle of 90 degrees is achieved. In the STN LCD panel, a twist angle increases to 180 to 360 degrees.
A high-density integrated circuit having a very fast response time has been used as a switching device for the LCD panel. A new mode for the LCD panel is required to keep up with the high-density integrated circuit. For the above-mentioned reason, a bistable twisted nematic (BTN) liquid crystal is introduced. The bistable twisted nematic liquid crystal is a nematic liquid crystal with a chiral dopant. A conventional BTN liquid crystal display panel is superior to the conventional simple matrix addressing liquid crystal panel such as an STN liquid crystal panel. For example, the BTN has characteristics of a wide viewing angle, a fast response time, and a high contrast ratio.
As shown in FIG. 3, in its first state, a conventional BTN liquid crystal 30 is aligned to have a twist angle of “φ” and be parallel to the substrates 2 and 4. With sufficiently high electric fields (hereinafter referred to as a reset electric field) applied across the BTN liquid crystal via a voltage source 40 of FIG. 4, the BTN liquid crystal becomes unstable in terms of an energy level, the boundaries between molecular layers are changed. Thus, disclination lines appear. Subsequently, the BTN liquid crystal 30 switches to the second state of a homeotropic alignment where molecules rotate and are perpendicular to the substrates 2 and 4. Therefore, with a reset electric field applied across the BTN liquid crystal 30, the BTN liquid crystal molecules arranges in the homeotropic alignment and becomes stable in terms of an energy level.
Thereafter, as shown in FIGS. 5A and 5B, when the reset electric field level is decreased, the BTN liquid crystal 30 quickly changes (switches) to the third state where the molecules are aligned with a twist angle of “φ−π” or “φ+π” according to decrease in the reset electric field. The above-mentioned third state having a twist angle of “φ−π” or “φ+π” is very different, in energy level, from the first state. However, it is the same as the second state in energy level. Since the third state is very different from the first state in its energy level, a disclination line is present between the first state and the third state and serves as an energy barrier. The energy barrier delays an inverted transition from the third state to the first state.
For the above-mentioned reason, the third state is the most stable. In addition, when another electric field is properly applied across the BTN liquid crystal in the third state, the BTN liquid crystal maintains its third state for a longer period of time. Specifically, if the reset electric field applied with the BTN liquid crystal is rapidly decreased, a backflow wave is induced, so that the BTN liquid crystal is aligned to be homeotropic with a first twist angle of Φ+π. Conversely, if the reset electric field is gradually decreased, the BTN liquid crystal is aligned to be homeotropic with a second twist angle of φ−π. A changing or switching speed of the BTN liquid crystal is less than 10 ms. In addition, as above-mentioned, when a proper electric field is additionally applied to the BTN liquid crystal in the third state, the third state lasts for more than 1 second.
FIG. 6 is a flow chart illustrating a switching operation of a LCD panel using a typical BTN liquid crystal. In the first state, the BTN liquid crystal has a twist angle of 180 degrees. When a reset voltage VR is applied to the BTN liquid crystal, the BTN liquid crystal arranges as a homeotropic alignment. As soon as the reset voltage VR is stopped, a selection voltage VS is applied to the BTN liquid crystal. If the selection voltage VS is higher than a saturation voltage Vsat, the BTN liquid crystal makes a transition to the first quasi-stable state where molecules are aligned with a first twist angle of 0 degree. Conversely, if the selection voltage VS is lower than the saturation voltage Vsat, the BTN liquid makes a transition to the second quasi-stable state where molecules are aligned with a second twist angle of 360 degrees. The first quasi-stable state becomes “ON” state while the second quasi-stable state becomes “OFF” state.
When a time-division operating voltage shown in FIG. 7 is applied to the above-mentioned BTN LCD panel, the first quasi-stable state switches to the second quasi-stable state, and vice versa. FIG. 7 shows a basic waveform of the operating voltage for the BTN LCD panel.
During a reset period TR, the reset voltage VR is applied such that the BTN liquid crystal changes from the first state alignment to the homeotropic alignment that is very different from the first state alignment in energy level. During a selection period TS, the BTN liquid crystal selects the first quasi-stable alignment or the second quasi-stable alignment according to the level of the selection voltage VS. During a retention period TR, the selected quasi-stable alignment lasts or a certain period of time with a retention voltage Vr applied with the BTN liquid crystal.
Specifically, in applying an actual voltage signal to a pixel electrode (reference 14 of FIG. 1), if a voltage higher than a threshold voltage is applied during the selection period, the BTN liquid crystal molecules are aligned to have the first twist angle of φ+π, which is OFF state. Conversely, if a voltage lower than the threshold voltage is applied during the selection period, the BTN liquid crystal molecules are aligned to have the first twist angle of φ−π, which is ON state. The higher selection voltage has the same effect as the reset voltage is gradually decreased, while the lower selection voltage has the same effect as the reset voltage is rapidly decreased. When the lower selection voltage is applied to the BTN liquid crystal in the second state, the backflow wave is induced.
As mentioned previously, if a twist angle of the first state is 180 degrees, the first twist angle of the first quasi-stable state, ON state, is 0 degree, while the second twist angle of the second quasi-twist state, OFF state, is 360 degrees. The above-mentioned ON and OFF states only refer to the different twist angles of the quasi-stable states. In other words, light transmittance or light reflectivity of the BTN LCD device may change according to some design factors of the BTN liquid crystal panel, such as cell thickness, refractive anisotropy, or angle of polarizers. Namely, according to the above-mentioned design factors of the BTN liquid crystal panel, ON and OFF states may respectively refer to white and black states, or black and white states.
As explained, the conventional BTN liquid crystal panel can switch ON and OFF states via changing only a twist angle, it is superior to the TN-LCD or STN-LCD panel in viewing angles as well as a response time and a contrast ratio. In addition, the conventional BTN liquid crystal is also operated via the simple matrix addressing operation.
However, a reset voltage of about 30 V is applied in the conventional BTN liquid crystal panel to make a transition from the first state alignment to the homeotropic alignment. Therefore, the conventional BTN liquid crystal panel has a disadvantage in a high power consumption. To overcome the above-mentioned problem, a homeotropic BTN liquid crystal panel using a vertical alignment layer was introduced. However, in this case, there is a limitation in which a liquid crystal should have a negative dielectric anisotropy.